Methane is a long-lived, well-mixed greenhouse gas. It has an atmospheric lifetime
of about 9 years. The tropospheric chemical models used to evaluate the impact
of the subsonic fleet found unanimously that CH4 lifetime
was reduced by aircraft emissions (see Chapter 4). This
instantaneous change (-1.3% in 1992, -2.6% in 2015, and -3.9% in 2050 for scenario
FSEGa (tech1)) needs to be increased further by a factor of 1.4 to include the
feedback of CH4 concentrations on lifetime (Prather,
1994; IPCC, 1996). It is then applied to the IS92a CH4
abundance to calculate the reduction in CH4 concentration
that can be attributed to aircraft. The CH4 perturbation
is assumed to be instantaneous; in reality, however, it takes a couple of decades
to appear and will lag the O3 perturbation. For the purposes
of interpolating between RF points in Table 6-1,
we assume that the CH4 perturbation, like the O3
perturbation, is proportional to NOx emissions.

6.3.5. Radiative Forcing for H2O

Water vapor is a potent greenhouse gas that is highly variable in the troposphere,
with a short average residence time controlled by the hydrological cycle. In
the stratosphere, the slow turnover of air and extreme dryness make precipitation
and clouds a rare phenomena, leading to smoothly varying concentrations ranging
from 3 to 6 ppmv as CH4 is oxidized to H2O
(Dessler et al., 1994; Harries et al., 1996). The contribution of aircraft to
atmospheric H2O is directly from the H in the fuel (assumed
to be 14% by mass). Most of the subsonic fleet's fuel is burned in the troposphere,
where this additional source of water is swamped by the hydrological cycle.
A smaller fraction is released in the stratosphere, where longer residence times
may lead to greater accumulation. However, because flight routes are close to
the tropopause and reach at most into the lowermost stratosphere, this effluent
is rapidly returned to the troposphere with little expected accumulation (Holton
et al., 1995; see also Section 3.3.4).

Although the uncertainty of predicting the current subsonic RF for water vapor
is large-a factor of 3-the absolute number in 1992 is estimated to be sufficiently
small, +0.0015 W m-2, making this factor a minor uncertainty
in subsonic climate forcing. It is assumed that this value scales linearly with
fuel use (see Table 6-1). This value is consistent
with earlier studies: Schumann (1994) and Fortuin et al. (1995) estimated that
present air traffic enhances background H2O by less than
1.5% for regions most frequently used by aircraft; likewise, Ponater et al.
(1996) and Rind et al. (1996) used GCM studies to conclude that the direct radiative
effect on the climate of water vapor emissions from 1992 air traffic is negligibly
small.

The projected HSCT fleet, however, would cruise at 20-km altitude and build
up much greater H2O enhancements in the stratosphere.
The stratospheric models described in Chapter 4 predicted
excess stratospheric water vapor from an HSCT fleet of 500 aircraft (designated
HSCT(500) in Table 6-1). This perturbation is difficult
to calculate, and the likely (2/3 probability) range includes a factor of 2
higher and lower. Furthermore, RF modeling of this stratospheric H2O
perturbation adds further uncertainty, as indicated in Table
6-1. All results suggest that this effect is the dominant HSCT climate impact,
with RF equal to +0.05 (0.017 to 0.15) W m-2 for 500
aircraft, increasing to +0.10 (0.03 to 0.30) W m-2 for
a mature fleet of 1,000 aircraft (HSCT(1000)). Although it takes several years
to accumulate this excess stratospheric water vapor, it is assumed that this
RF is instantaneously proportional to the HSCT fleet size.

In a GCM study, Rind and Lonergan (1995) looked for climate change caused by
H2O accumulation from a fleet of 500 HSCTs. They found
no statistically significant change in surface temperature. Their result is
consistent with this assessment and with water vapor as the dominant HSCT climate
impact because the magnitude of this radiative forcing from the fleet, +0.05
W m-2, would induce a mean global warming that would
be difficult to detect above natural climate variability.